1. Field of the Invention
Embodiments of the present invention relate generally to image sensors and methods and, in specific embodiments, to image sensors that allow for antiblooming to drain excess charge from photodiodes and that allow for the driving of control signals to pixels.
2. Related Art
Image sensors have found wide application in consumer and industrial electronics, and have enabled an explosion in a number of digital cameras and digital video devices used for work and entertainment. In many applications, and especially in industrial applications, there is a constant demand for image sensors with faster processing speed and better image quality. Thus, it is advantageous to develop new architectures that allow for improved performance of image sensors.
FIG. 1 illustrates an architecture of a related art image sensor 1. As illustrated in FIG. 1, the image sensor 1 comprises a pixel array 8 and a row driver 29. The pixel array 8 comprises pixels 2 that are arranged in rows and columns. Each pixel 2 comprises a light sensitive element, such as a photodiode, or the like, to sample light intensity of a corresponding portion of a scene being imaged, and each pixel 2 is configured to produce an analog pixel signal based on the sampled light intensity. The row driver 29 supplies control signals to the pixels 2 in the pixel array 8 to control an operation of the pixels 2.
Pixels 2 that are in a same row of the pixel array 8 share common row control signals from the row driver 29. For example, pixels 2 in a first row of the pixel array 8 share common row control lines 211 for receiving control signals from the row driver 29. Similarly, pixels 2 in a second row of the pixel array 8 share common row control lines 212 for receiving control signals from the row driver 29, and pixels 2 in an hth row of the pixel array 8 share common row control lines 21h for receiving control signals from the row driver 29. Pixels 2 that are in a same column of the pixel array 8 may share a common column readout line to provide output. For example, pixels 2 in a first column of the pixel array 8 share a column readout line 221, pixels 2 in a second column of the pixel array 8 share a column readout line 222, and pixels 2 in an m column of the pixel array 8 share a column readout line 22m. The row driver 29 typically controls the pixels 2 to provide output row by row.
Further examples of related art image sensors are disclosed in the following references: (i) U.S. Pat. No. 6,870,565 entitled “Semiconductor Imaging Sensor Array Devices with Dual-Port Digital Readout”, the entire contents of which are incorporated by reference herein; (ii) U.S. Patent App. Pub. No. 2003/0043089 entitled “Doubling of Speed in CMOS Sensor with Column-Parallel ADCs”, the entire contents of which are incorporated by reference herein; and (iii) A. Krymski et al., “A High Speed, 500 Frames/s, 1024×1024 CMOS Active Pixel Sensor”, 1999 Symposium on VLSI Circuits Digest of Technical Papers, 1999, Kyoto, Japan, pp. 137-138, the entire contents of which are incorporated by reference herein.
FIG. 2A illustrates an example design of the pixel 2. The pixel 2 in FIG. 2A is typically called a four transistor (4T) pixel. The pixel 2 includes a photodiode 11, a transfer transistor 112, a sense node 13, a reset transistor 114, a source follower transistor 116, and a row select transistor 118. The transfer transistor 112, the reset transistor 114, the source follower transistor 116, and the row select transistor 118 may each comprise, for example, an n-channel metal-oxide semiconductor field effect transistor (NMOS transistor), or the like.
The pixel 2 illustrated in FIG. 2A is provided as an example of a pixel in an ith row and a jth column of a pixel array, such as the pixel array 8 (refer to FIG. 1), and the pixel 2 receives a transfer signal (tx) over a transfer signal line 21i1, a reset signal (rst) over a reset signal line 21i2, and a row select signal (rowsel) over a row select signal line 21i3. The transfer signal line 21i1, the reset signal line 21i2, and the row select signal line 21i3 are shared by all pixels in an ith row of a pixel array, such as the pixel array 8 (refer to FIG. 1), and the transfer signal (tx), the reset signal (rst), and the row select signal (rowsel) are provided from a row driver, such as the row driver 29 (refer to FIG. 1). The pixel 2 in FIG. 2A provides output to a column readout line 22j.
As illustrated in FIG. 2A, an anode of the photodiode 11 is connected to a fixed voltage, such as gound or another suitable voltage, and a cathode of the photodiode 11 is connected to a source of the transfer transistor 112. A gate 12 of the transfer transistor 112 is connected to the transfer signal line 21i1, and the gate 12 of the transfer transistor 112 may also be called the transfer gate 12. A drain of the transfer transistor 112 is connected to the sense node 13. A source of the reset transistor 114 is connected to the sense node 13, and a drain of the reset transistor 114 is connected to a supply voltage (Vdd) provided from a power supply (not shown). A gate 14 of the reset transistor 114 is connected to the reset signal line 21i2, and the gate 14 of the reset transistor 114 may also be called the reset gate 14.
A drain of the source follower transistor 116 is connected to the supply voltage (Vdd) provided from the power supply (not shown), and a source of the source follower transistor 116 is connected to a drain of the row select transistor 118. A gate 16 of the source follower transistor 116 is connected to the sense node 13, and the gate 16 of the source follower transistor 116 may also be called the source follower gate 16. A source 19 of the row select transistor 118 is connected to the column readout line 22j, and the source 19 of the row select transistor 118 may also be called the row select transistor source 19 or the pixel output area 19. The pixel output area 19 provides a pixel output (pout) signal to the column readout line 22j. A gate 18 of the row select transistor 118 is connected to the row select signal line 21i3, and the gate 18 of the row select transistor 118 may also be called the row select gate 18.
FIG. 2B illustrates a top-down view of an example layout of the pixel 2 of FIG. 2A. With reference to FIGS. 2A and 2B, a portion of a diffusion for the photodiode 11 may extend to serve as the source of the transfer transistor 112. Also, the sense node 13 may include a common diffusion that serves as the drain of the transfer transistor 112 and as the source of the reset transistor 114. The transfer gate 12 serves as a gate between the photodiode 11 and the sense node 13. A common diffusion 15 may serve as the drain 15 of the reset transistor 114 and also as the drain 15 of the source follower transistor 116. The reset gate 14 serves as a gate between the common diffusion 15 and the sense node 13. A common diffusion 17 may serve as the source 17 of the source follower transistor 116 and also as the drain 17 of the row select transistor 118. The source follower gate 16 serves as a gate between the common diffusion 15 and the common diffusion 17. The common diffusion 15, which serves as the drain 15 of the source follower transistor 116, is connected to the supply voltage (Vdd) provided from the power supply (not shown). The row select gate 18 serves as a gate between the common diffusion 17 and the pixel output area 19. FIG. 2C illustrates a top-down view of a portion 4 of a pixel array, such as the pixel array 8 (refer to FIG. 1). With reference to FIGS. 1 and 2C, the portion 4 of the pixel array 8 includes two pixels 2 that are adjacent to each other in a row of the pixel array 8.
Two types of shutter functions that are commonly employed in image sensors are (i) a global shutter operation; and (ii) a rolling shutter operation. In a typical global shutter operation, all pixels in a pixel array are reset, and then exposure is started simultaneously in all of the pixels in the pixel array, and then exposure is ended simultaneously in all of the pixels in the pixel array. In the typical global shutter operation, the charges stored in the pixels of the pixel array after exposure has ended are then read out row-by-row from the pixel array. In a typical rolling shutter operation, exposure starts at a same time for all pixels in a same row of the pixel array and ends at a same time for all pixels in a same row of the pixel array, but a time at which exposure starts is staggered for different rows in the pixel array. Rolling shutter operations have some disadvantages when capturing images of moving objects, because exposure starts in different rows at different times. Global shutters are better suited for capturing images of moving objects since exposure starts in all pixels at a same time, but global shutters may lead to problems in traditional 4T pixels in that stored charges in pixels may be compromised while waiting for readout, as will now be discussed in more detail.
An operation of the pixel 2 in FIG. 2A during a global shutter operation may proceed as follows: (i) a row driver, such as the row driver 29 (refer to FIG. 1) provides a HIGH signal on the reset signal line 21i2 and a HIGH signal on the transfer signal line 21i1 to discharge the photodiode 11 and reset the sense node 13; (ii) the row driver provides a LOW signal on the reset signal line 21i2 and a LOW signal on the transfer signal line 21i1 to close the reset gate 14 and the transfer gate 12 for an exposure time during which the photodiode 11 accumulates charge from sensed light; (iii) the row driver provides a HIGH signal on the transfer signal line 21i1 to open the transfer gate 12 to transfer the accumulated charge from the photodiode 11 to the sense node 13; (iv) the row driver provides a LOW signal on the transfer signal line 21i1 to close the transfer gate 12 to hold the charge at the sense node 13 waiting for a readout of charge from the row in which the pixel 2 is located; and (v) when charge from the row in which the pixel 2 is located is to be read out, the row driver provides a HIGH signal on the row select signal line 21i3 to open the row select gate 18 to provide output to the column readout line 22j.
Thus, in a global shutter operation, charge transferred to the sense node 13 of the pixel 2 is held waiting for readout as the readout proceeds from row to row. This may work in the case of a full-frame shutter for a global shutter operation when the shutter is open from one transfer to another. However, short shutters for global shutter operations might not be practical, because after the charge transfer to the sense node 13 is completed and the transfer gate 12 is closed, the photodiode 11 continues to accumulate photocharge and can get easily overfilled, which would cause excessive charges to leak from the photodiode 11 to the sense node 13 over the closed transfer gate 12 and compromise the useful charge that had been shuttered to the sense node 13 and that was waiting for readout.
One way that has been developed to deal with the problem of excess charge from a photodiode leaking over a closed transfer gate to a sense node is to add an additional transistor to the traditional 4T pixel of FIG. 2A to create a traditional five transistor (5T) pixel, where the additional transistor is used as an antiblooming transistor for draining excess charge from a photodiode. FIG. 3 illustrates a layout of a conventional 5T pixel 55. The pixel 55 is similar to the pixel 2 (refer to FIG. 2A), except that the drain of the reset transistor 114 is connected to a reset supply voltage Vrst rather than to Vdd, and an antiblooming transistor 60 is added between the cathode of the photodiode 11 and the reset supply voltage Vrst. A gate of the antiblooming transistor 60 is connected to a control signal (sh). In the pixel 55, when charge has been transferred to the sense node 13 and the charge in the sense node 13 is waiting for readout, a HIGH signal may be provided to the antiblooming transistor 60 to drain excess charge from the photodiode 11 through the antiblooming transistor 60. In some instances, the control signal (sh) may be LOW only for the exposure time during which the photodiode 11 accumulates charge from sensed light and for the transfer time. The conventional 5T pixel 55 of FIG. 3 has disadvantages with respect to the conventional 4T pixel 2 of FIG. 2A in that the 5T pixel 55 is more difficult to lay out in a small size due to the additional transistor that is needed and due to two more lines that are required for control and bias.
Another issue of concern to image sensor designers is a maximum frame rate of an image sensor. The trend in image sensors is to have image sensors with more pixels, higher frame rates, and tighter design rules. With reference to FIG. 1, a signal delay for a signal to propagate from the row driver 29 across control lines, such as the control lines 211, to all of the pixels in a row may be dominated by resistive-capacitive effects of the control lines. Due to the trends in image sensors, control lines such as the control lines 211 are becoming longer, thinner, and need to operate faster. As an example of resistance and capacitance figures for a control line, a 20 mm long control line made in the first aluminum of 0.2 μm width and having resistivity of 120 mOhm/square, has resistance of 12 kOhm and an approximate capacitance of 2 pF. Thus, the characteristic control delay time of such a control line would be approximately 12 kOhm*2 pF, which is 24 ns. Such a delay time could set a limit on an achievable frame rate for an image sensor.